I would go with biochemistry. (1). I find biochem harder than physiology. (2). I am not sure about this, but I don't think a single med school requires physiology for admission, but there are some med schools that do require biochemistry for admission. (3). Most likely, I am guessing, you will start of with biochem.
But enjoy this summer more than anything. But yes, if you have absolutely honestly nothing to do (and can't even get to sleep), than reviewing biochem won't hurt.